Reactive Strength as a Performance Measure

Reactive strength may be one of the most powerful indicators of athleticism, as it represents the ability to effectively make use the stretch shortening cycle (SSC). It is an expression of the ability to change from an eccentric to a concentric contraction as quickly as possible and with as much force as possible. This type of effort happens quite repetitively in competition as athletes move across the field, court, track, etc. While the prevalence and importance of reactive strength should seem obvious, application can be lost without good understanding of what is happening. Here, we will look to delve thoroughly into many aspects of the topic as we travel from a mechanical and physiological analysis, to thoughts on how, when, and why to test, and finish with some important training considerations.

Mechanisms of Reactive Strength

Stiffness

Tendon stiffness is a determinant of the quality of a fast stretch shortening cycle

Tendon stiffness is an important property in high-level SSC action [4, 6]. While not the only factor at play, the stiffness of the tendon involved in the joint loading is a determinant of how much and how efficiently energy can be absorbed, stored, and released into propulsion. For example, when the foot hits the ground, the Achilles begins to stretch like a rubber band. A new rubber band with greater stiffness will be more resistant to stretch and will return to original form much quicker than an old worn out band. In the same way, tendons will deform less and react more powerfully when possessing stiffness versus compliance. Tendon stiffness differs somewhat from muscle-belly stiffness, as the former is a product of collagen quality and density, and fascial fiber orientation [10], while muscle stiffness is largely a neuromuscular quality. Stiffer tendons may stretch to the same length as compliant ones, but will shorten at a much higher velocity due to the increased elastic energy stored during stretch [4]. For this reason, increasing tendon stiffness alone can improve the quality of a stretch-shortening cycle without increase in muscular strength.

Neuromuscular Efficiency

While the role of the tendon in improved reactive strength is largely due to its mechanical properties, the contractile elements of the muscle-tendon unit contribute through a different pathway. Hiyarama et al. found that a group training for reactive strength expressed a switch from concentric to isometric contraction in the triceps surae (agonist) during the initial phase of propulsion in a depth jump. This isometric contraction was essentially a prolonged ‘stiffening’ of the musculature, allowing the tendon to release its elastic energy prior to muscle action taking over. The greater reliance on the tendon allowed for faster initiation of propulsion and more efficient force generation. The study also showed a decrease in muscle activation of the antagonist, and an increase in muscle activation of the agonist during the braking phase of landing. Both contributed to a more efficient eccentric contraction that optimizes tendon load going into the amortization. This evidence supports improved neuromuscular control of the system, as changes in contraction timing, type, and magnitude again elicited greater performance on depth jumps even with no significant improvement in maximal isometric strength from the plantar flexors [4].

It seems clear that there is some neuromuscular ‘learning’ associated with reactive strength training, especially in terms of the efficiency of the muscular contractions involved in a stretch-shortening cycle. Evidence also points to the stiffness of the tendon as a necessary component of the effective SSC, such that it is relied upon more heavily as neuromuscular efficiency improves.

Testing

The most common way to test reactive strength is through protocols that yield a Reactive Strength Index (RSI). This value is simply a ratio of either jump height to contact time, or flight time to contact time. RSI testing comes in a few forms each holding their own power, but remains a relatively simple tool for assessing the ability of an athlete to apply large forces over a very short amount of time. Two of the most common protocols are the incremental-height Drop Jump (DJ) test, and the 10-5 test of repeated jumps. While I personally see the DJ test more steadily used in research, I chose to also include the 10-5 protocol here to show the different qualities the two tests can describe in an athlete.

Each protocol involves an athlete jumping maximally on and off of either a force plate or contact mat, or using a wearable velocity measurement tool like a Push Band. In the incremental-height DJ test, athletes fall off of a box and perform one jump of maximal height and minimal ground contact time. This is then repeated for higher boxes until the best RSI value is achieved. In contrast, the 10-5 jump test is done by performing 10 consecutive jumps, also of maximal height and minimal ground contact time, where the final RSI is either the average of the 5 best or the average of the last 5 jumps [2]. Both tests aim to keep ground contact below.25 seconds as a standard for a fast stretch shortening cycle.

Figure 1: RSI thresholds for the Drop Jump [4].

Information from both protocols can help to create a profile of reactive strength ability for the athlete. The DJ test gives insight into the athlete’s eccentric force generating capacity. Athletes who achieve their best RSI at relatively shorter box heights likely display stiffness, but don’t possess the fast eccentric strength necessary to maintain short contacts and produce long flight times off of higher boxes. Conversely, athletes who excel at relatively larger box height will also display good stiffness, but their reactive strength capability actually benefits from a greater pre-load from a higher fall to achieve a bigger jump. Research actually promotes the DJ protocol as a determinant of proper box height for training the depth jump [1]. The most appropriate set-up for maximizing potential stimulus would be the one where the athlete achieved their best RSI.

The 10-5 testing protocol may be better suited for assessing stiffness because the drop height is constrained by the jump height of the previous repetition. If an athlete reaches their maximum height for a set of repetitive jumps, the only way to improve the RSI is to get off the ground faster on the next jump and maintain flight time. This rep-to-rep improvement in stiffness is not due to a change at the tendon but a coordinated neuromuscular effort. However, as was mentioned earlier, an improvement in contraction efficiency puts greater reliance on the tendon to express its stiffness. The multi-jump approach to testing RSI allows for this type of adjustment. Athletes often achieve their best score toward the end of the 10 jumps as they find a rhythm, which gives further information that a single effort on the DJ test may not.

Why Test?

The two main reasons to use any test are to measure improvements and to predict performance outcomes. Research by Randall Jensen on rugby players found RSI to be the best predictor of 30-meter sprint performance above countermovement jump, squat jump, rebound jump, and back squat [5]. Other studies [1,4,7] show a more general ability of RSI to measure the effectiveness of the stretch-shortening cycle, which is applicable to a majority of athletic movements. The evidence seems to suggest that RSI is capable of predicting at least some performance outcomes and, as an indicator of reactive strength improvements, is as reliable as the technology used to test it.

Because of its dependence on two variables, RSI is sensitive to worthwhile changes in performance

Because the RSI value is dependent upon two variables (contact time and flight time), it is quite sensitive to worthwhile changes in performance. A slight change in either variable will have a compounded effect on the RSI value [7]. We learned earlier that reactive strength is highly dependent on tendon stiffness and neuromuscular control of the attached muscles. This means that managing fatigue of the central nervous system may show its effects in RSI scores. It is because of this that many performance specialists are using RSI testing as an athlete monitoring system to essentially measure neuromuscular fatigue and predict readiness [8]. Daniel Martinez suggested, from a thorough review of RSI, that its use as a readiness monitor may be best suited for training phases where high elastic demand is the primary focus [7].

If RSI can be used as a window into the presence of fatigue, it should also be effective in showing the lack thereof. Eamonn Flannagan proposed the use of RSI as a monitoring system for evaluating the effectiveness of a peaking phase of training [2]. This is a time when volumes are dropping, arousal is heightened, and the readiness of the system (body) should be at its peak. During this phase, RSI monitoring hopes to show a trend upward in scores if the overall load on the body is being properly managed.

Regardless of when it’s used, RSI as a monitoring tool relies on baseline data as well as consistent measurements throughout the training. It is necessary to see the trends as a result of different training phases as well as the differences in how each athlete responds to training and how that response is expressed physiologically. Creating a complete athlete profile is the only way to make good informed decisions based on the data collected.

Training Considerations

Reactive strength is a piece of an athlete profile, reliant on certain qualities and predictive of others. While we know that reactive strength is important for athletic movements, we should also want to know how to improve it. The role of maximum strength in the development of reactive strength comes down to improvements in rate of force development (RFD) and eccentric force capacity. While eccentric force, relative to explosive movements, is influenced by timing and coordination of fast eccentric contractions, the capacity of the tissue to purely produce force is underlying. Similarly, the expression of RFD is dependent on the capacity for initial force production [9]. Put simply, faster forces can be produced if more total force is available. Jensen et al. notes that when the time available for the SSC is less than .3 seconds, RFD is more of a deciding factor in performance than max strength [5], but others indicate that resistance training supports the development of RFD [9]. For this reason, we can regard max strength as a catalyst for improving explosive efforts through increased RFD, and a necessary foundation to the ballistic type of training necessary to improve fast eccentric force capacity and thus reactive strength. What we are left with is basically practicing that which we hope to develop. Doing fast SSC plyometrics is a clear and direct path toward improving reactive strength. Doing so in a sensible and progressive manner is key. Someone with well-established stiffness during low-level activity may be ready to progress on to higher load eccentric landings to develop the ability to maintain stiffness during high-intensity activity. Another athlete may lack fundamental tendon stiffness and would initially benefit from higher volumes of low intensity repetitive contacts. Either of these scenarios could be highlighted by way of the RSI testing protocols mentioned earlier.

Keep in mind, also, that these RSI tests are standardized to take place in the sagittal plane. Most team sports, however, spend little time isolated in any one plane during explosive movement. For this reason, it is important to understand that a test of bilateral sagittal plane reactive strength is an indicator of a general SSC capability in this context. When training athletes for the rigors of team sport, the use of unilateral explosive efforts and multi-planar SSC contractions will become necessary throughout training. These movements require a specific development of stiffness and eccentric force generating capacity as well as balance and coordination [3]. A rule of thumb is to use RSI to monitor the system and as a general guide through the exploration of more complex plyometrics.

No matter the need of the athlete, there are a few important considerations in progressing plyometric activity for improving reactive strength.

Tendon stiffness underlies all levels of reactive strength expression

Load absorption capacity (Depth Drop) should be developed before elastic energy storage and release (Depth Jump)

Plyometric intensity is determined by height of fall, stiffness of contact, single leg vs. bilateral, and the presence of horizontal movement (see Mike Young’s post on mechanical model for plyometric classification).

Keeping a good balance between contact time and flight time ensures appropriate load and adequate stimulus (specific to faster SSC plyos).

Neuromuscular efficiency may improve quickly, but tendons adapt more slowly. Be patient in advancing load.

To wrap up and quickly recap; the underlying mechanisms of reactive strength are tendon stiffness and a neuromuscular ‘learning’ toward proper contraction type, timing, and intensity. High levels of reactive strength are not only stiff and efficient, but also backed up by maximal strength ability, specifically eccentric force generating capacity and rate of force development. Training for reactive strength can take many forms, but a few key principles can keep coaches on the right track. Finally, testing for Reactive Strength Index is a reliable way to measure progress, and also shows validity in predicting performance outcomes like 30-meter sprint time. Furthermore, RSI testing has plausible use as a readiness-monitoring tool because of its sensitivity to changes in performance. Reactive strength should be the topic of many conversations as the pool of research grows showing its power as a predictor of high performance activity.

Greg Gustin

Greg Gustin is the Assistant Performance Director at Athletic Lab and the Assistant Fitness Coach for the North Carolina Courage and North Carolina FC. Greg has a BS in Exercise Science and MS in Health and Fitness. Greg is recognized as a Certified Strength and Conditioning Specialist (CSCS) by the National Strength and Conditioning Association and a Level 1 Coach by USA Track & Field.

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